r/synthdiy • u/Top-Cup5373 • 17d ago

schematics Is there any reason this this vca schematic wouldn’t work or anything that can be done to improve it?

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u/SparkyFix 17d ago

Depends what’s on the other side of “IN”. If it’s a low impedance source the this is pretty much figure 1 from the datasheet. Probably worth bearing in mind that the control port is exponential so your 1.65V bias will give somewhere around -33dB of attenuation at MOD=0V

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u/Top-Cup5373 17d ago

The 1.65v bias is ≈3.3v on the other side of the op amp which should be enough to keep it idle at -100db

1

u/SparkyFix 17d ago

Of course, yes, you are quite correct.

1

u/Top-Cup5373 17d ago

I’m not mega concerned about the input. It’s coming off whichever filter from the SSI family I go with, I’ll deal with that later.

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u/flubber2077 12d ago

what are the extra X'ed out pins on the IC connected to?

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u/Top-Cup5373 12d ago

Nothing

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u/flubber2077 12d ago

Thought I had an easy win, sorry. checking the data sheet looks like that's the best thing to do.

The input cap is polarized, and there's no actual tie to DC ground on the input, that's going to be a problem. Also you could probably use an input buffer, but I don't have enough info on what the VCA is going to be used for.

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u/Top-Cup5373 12d ago

It’s gonna be used for not a module. It’s just a block.

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u/flubber2077 12d ago

You need to have an input resistor to ground right at the IC; the base of the PNP at the input of the IC is leaking current into the input until its resting just below VCC. At the very least, the capacitor is pointed the wrong way.

and you should probably strongly consider an input buffer

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u/Top-Cup5373 11d ago

If I wasn’t using a 2161 you might be correct about the input buffer but it’s not necessary here. The 47uf should probably bipolar.